This invention relates generally to nuclear reactors, and more particularly to jet pump slip joint clamp apparatus for boiling water nuclear reactors.
A reactor pressure vessel (RPV) of a boiling water reactor (BWR) typically has a generally cylindrical shape and is closed at both ends, e.g., by a bottom head and a removable top head. A top guide typically is spaced above a core plate within the RPV. A core shroud, or shroud, typically surrounds the core and is supported by a shroud support structure. Particularly, the shroud has a generally cylindrical shape and surrounds both the core plate and the top guide. There is a space or annulus located between the cylindrical reactor pressure vessel and the cylindrically shaped shroud.
In a BWR, hollow tubular jet pumps positioned within the shroud annulus, provide the required reactor core water flow. The upper portion of the jet pump, known as the inlet mixer, is laterally positioned and supported against two opposing rigid contacts within restrainer brackets by a gravity actuated wedge. The restrainer brackets support the inlet mixer by attaching to the adjacent jet pump riser pipe. The lower portion of the jet pump, known as the diffuser, is coupled to the inlet mixer by a slip joint. The slip joint between the jet pump inlet mixer and the jet pump diffuser collar has about 0.015 inch diametral operating clearance which accommodates the relative axial thermal expansion movement between the upper and lower parts of the jet pump and permits leakage flow from the driving pressure inside the pump.
Excessive leakage flow can cause oscillating motion in the slip joint, which is a source of detrimental vibration excitation in the jet pump assembly. The slip joint leakage rate can increase due to single loop operation, increased core flow, or jet pump crud deposition. The restrainer bracket laterally supports the inlet mixer through three point contact provided by two set screws and the inlet mixer wedge at an elevation above the slip joint. Set screw gaps can occur during plant operation. Sometimes, the inlet mixer appears to settle to a position away from the set screw, while in other cases, wear occurs between the mixer wedge and the restrainer pad. In both cases, three point contact is lost and the potential for vibration is significantly increased. Set screw gaps are affected by the difference in thermal and pressure displacements of the shroud, pressure vessel, and rotation of the shroud support plate. In addition to affecting set screw gaps, thermal and pressure displacements of the shroud and the pressure vessel can diminish alignment interaction loads in the jet pump assembly which are beneficial in restraining vibration, such as a lateral force in the slip joint. The resultant increased vibration levels and corresponding vibration loads on the piping and supports can cause jet pump component degradation from wear and fatigue.
High levels of flow induced vibration (FIV) are possible in some jet pump designs at some abnormal operational conditions having increased leakage rates. Therefore, it is desirable to provide a jet pump assembly that that has a lateral load in the slip joint area to maintain rigid contact between the inlet mixer and diffuser collar to prevent oscillating motion and suppress FIV.
In an exemplary embodiment, a clamp apparatus for a jet pump slip joint in a boiling water nuclear reactor pressure vessel includes a C-yoke. The C-yoke includes an arcuate main portion, an engagement portion extending from a first end of the main portion, and a locking preload portion extending from a second end of the main portion.
The engagement portion includes an engagement slot configured to receive a diffuser guide ear, and transmit a clamp preload force onto the jet pump diffuser. The locking preload portion includes a threaded clamp bolt opening extending therethrough, and a clamp bolt threadedly engaging and extending through the clamp bolt opening. The clamp bolt is configured to engage an outer surface of the inlet mixer and includes a locking collar having a plurality of ratchet teeth spaced around the periphery of the locking collar. A hexagonal drive extension extends through the locking collar. The hexagonal drive extension is used to tighten the clamp bolt to apply a predetermined preload to the jet pump diffuser. A locking spring is coupled to a surface of the locking portion adjacent the bolt opening. The locking spring engages the ratchet teeth of the locking collar to lock the clamp bolt in place and prevent the bolt from loosening.
The above described clamp apparatus provides a lateral load in the slip joint area of a jet pump to maintain a tight and rigid contact between the inlet mixer and diffuser to prevent oscillating motion and suppress FIV. The clamp apparatus is remotely installable, requires minimum installation time, and does not require disassembly of the jet pumps.